Using a yeast two-hybrid genetic screen we have isolated a novel cDNA that codes for a protein named XBP (X11 Binding Protein), which interacts with the PDZ region of X11. X11 is a brain specific protein interacting with the beta-amyloid precursor protein (APP). The putative carboxyl terminus of XBP contains a four amino acid motif - GTTV, matching a consensus sequence required for interaction with PDZ domains. XBP bears some sequence similarity to the moesin/ezrin/radixin family of proteins that cross-link and regulate actin-plasma membrane interactions. We hypothesize that XBP is a crosslinking adaptor-like protein which plays a role in organizing a multi-protein complex involving APP. XBP may participate in both structural and regulatory interactions of APP at the plasma membrane. Since X11 and another APP- interacting protein, Fe65 bind to the NPTY internalization motif on APP, it is likely that their interactions with other protein ligands will have direct consequences for the APP function. Given that the NPTY motif is a tyrosine-based internalization signal required for degradation of APP, specific interactions within the APP multi-protein complex may directly affect proteolytic processing of APP, and result in generation of beta-amyloid peptide (Abeta). Whether Abeta itself or rather aberrant processing of its precursor is responsible for AD pathogenesis, detailed knowledge of the APP organization at the plasma membrane is necessary to fully understand the molecular pathways of the disease. The overall goal of this research is to gain insight into the structural and functional interactions of APP with other neuronal proteins that may lead to elucidation of the normal function of APP in the brain. This proposal deals with a fundamental aspect of the APP function and regulation. Its objective is to extend our preliminary by documenting that XBP specifically interacts with X11. We propose to study the XBP protein by characterizing its ability to interact with functional fragments of X11 both in vitro and in vivo by using recombinant fusion proteins and specific antibodies.